1) Field of the Invention
The present invention relates to an optical module, and more particularly, to an optical module suitable for use with an optical transmitter employed in an optical communications system.
2) Description of Related Art
An optical network has recently been used as a high-capacity transmission network. From the viewpoint of an increase in a transmission distance, an external modulation system, which modulates the intensity or phase of a CW (Continuous Wave) light source by means of an external modulator, has been widely known in the field of this optical network. With an aim of miniaturizing an optical transmitter, a transmitter module into which the function of the light source and that of the external modulator are integrated has been sought.
A wavelength multiplex technique for multiplexing a plurality of light beams of different wavelengths and transmitting the thus-multiplexed light beam through a single optical fiber is available as a technique for increasing the channel capacity of the optical network. However, if production variance exists in laser oscillation wavelength, difficulty will be encountered in obtaining a light source of desired wavelength. Further, the wavelength multiplexing technique entails a necessity for preparing light sources conforming to respective wavelengths used in wavelength multiplex optical communication.
As a result of diversification of specifications of the light sources having stable light emission wavelengths, there sometimes arises an inventory problem. For this reason, a light source—which enables variable adjustment of the wavelength of a laser to be oscillated—is sought. A laser light source of external oscillator type, which enables variation of the wavelength of laser oscillation by means of varying oscillation requirements, has been put forward as a wavelength-adjustable light source. An example laser light source of external oscillator type is shown in FIG. 10.
A laser light source 100 of external oscillator type shown in FIG. 10 comprises a gain member 101 having at one end an antireflective film 101a (an AR coating) and at the other end a partial reflection mirror 101b; and a total reflection mirror 102 disposed opposite the antireflective film 101a with a space interposed therebetween. By means of this configuration, light spontaneously emitted from the gain member 101 propagates along a two-way optical path between the partial reflection mirror 101b and the total reflection mirror 102, whereby a laser beam oscillation is induced.
Oscillation requirements for the two-way optical path are determined according to the distance between the partial reflection mirror 101b and the total reflection mirror 102. As mentioned above, a section of optical path between the total reflection mirror 102 and the partial reflection mirror 101b serves as a variable external oscillator 102A which enables variable provision of oscillation requirements. Specifically, the wavelength of light to be oscillated can be set by means of actuating the total reflection mirror 102 so as to set the distance between the total reflection mirror 102 and the partial reflection mirror 101b. 
At the time of manufacture of the optical transmitter module adopting the above-described external modulation system, the practice of integrally manufacturing a wavelength-variable laser light source of external resonator type employed as a CW light source and an external modulator is not carried out. The optical transmitter module has hitherto been manufactured by a method of separately fabricating the laser light source and the external modulator and connecting them together through butt-joining. However, this manufacturing technique has encountered difficulty in aligning the light source with the external modulator, and involves a relatively heavy workload. Further, in the event of a loss caused by misalignment inducing a decrease in output, there will arise a problem of an individual difference arising in output intensity.
To solve the problem, an integrated laser light source of external type is described in Patent Document 1, which will be described below, as a configuration which does not need alignment operation during butt-joining and into which the wavelength-variable light source and the external modulator are integrated. As shown in, e.g., FIG. 11, the integrated external resonator laser light source described in Patent Document 1 is configured by means of integrating the wavelength-adjustable external resonator laser 100 and an external modulator 104.
Specifically, the integrated external resonator laser light source 110 is configured by means of arranging a partial reflector 103 and the external modulator 104 side by side in an optical path and integrally with the gain member 101 constituting the laser light source 100 of external resonator type analogous to that shown in FIG. 10. A reflector of etched facet type or a waveguide loop mirror is applied as the partial reflector 103 formed integrally with the gain member 101 and the external modulator 104. Reference numeral 105 designates a light waveguide which serves as a light propagation path penetrating through the gain member 101, the partial reflector 103, and the external modulator 104, all of which are formed integrally.
Techniques described in Patent Documents 2 to 5 are also available as known techniques relevant to the present invention. Patent Document 2 describes an integrated semiconductor laser where a gain region and a waveguide region of a mirror, which is optically coupled to one end face of a directional waveguide, are integrated on a semiconductor substrate. An external oscillator of a semiconductor laser described in Patent Document 3 has a directional coupling waveguide having a total reflection mirror at one waveguide end face thereof. Moreover, Patent Document 4 describes a technique of integrally forming a conductor light absorbing region and a variable light attenuation region. Patent Document 5 describes a wavelength selection element where a light reflection section is provided at either end of a directional coupling waveguide.
(Patent Document 1) JP-2003-508927A
(Patent Document 2) JP-SHO-64-25587A
(Patent Document 3) JP-HEI-2-114691A
(Patent Document 4) JP-2000-77771A
(Patent Document 5) JP-HEI-5-45681A
However, as described in Patent Document 1, when a partial reflector of facet type is fabricated between a gain member and an external modulator by means of etching, the reflector assumes a complicated structure. The current device manufacturing technique encounters difficulty in manufacturing a partial reflectivity capable of acquiring sufficient reflectivity. There may also arise a case where limitations are imposed on a wavelength range where partial reflection is achieved or a case where multiple reflection arises because the reflector has a plurality of reflection faces. The multiple reflection may hinder an improvement in stability of operation of the resonator.
As described in Patent Document 1, when a partial reflector consisting of a waveguide loop mirror is provided, a waveguide having a gentle curve must be formed to form a waveguide for the loop area such that the loss of the waveguide is reduced with a view toward enhancing oscillation efficiency. Therefore, there also arises a problem of the module becoming bulky.
The techniques described in Patent Documents 2 to 5 do not describe the laser light source integrated with an external modulator which solves the above problem.